While the bride wore a custom-made Marchesa dress, the guests – which included Robert Downey Jr., model Coco Rocha and Levine's bandmates James Valentine and Jesse Carmichael – were not held to a specific dress code, according to a source.

Still, their loved ones came dressed for the star-studded affair – and the hot weather – with most of the women donning summer-inspired dresses and many of the men opting for linen suits.

"All the guests were happy and relaxed," says another source. "They were excited to celebrate with the couple on their big day."

Levine, 35, partied with a group of about 50 friends at the Palms Casino Resort in Las Vegas for his bachelor party in March, but the groom recently told PEOPLE he "had enough bachelor parties" in his lifetime and was ready to settle down with Prinsloo, 25.

"I definitely feel like I'm sitting in the chair I'm supposed to be sitting in right now," he said. "It all feels very natural."